Poly buffered local oxidation of silicon isolation technology uses a poly silicon film sandwiched between a thermal silicon oxide and a silicon nitride film to relieve stress during substrate oxidation. The stress relief is important to minimize stress induced defects such as dislocations in the silicon substrate. Poly buffered LOCOS technology also offers a number of other advantages, such as shorter bird's beak length.
The integrity of a poly silicon layer during subsequent high temperature processing, such as field oxidation, is crucial to the successful manufacturing of devices with poly buffered LOCOS isolation technology. Pits are frequently observed in the poly silicon layer after the removal of the silicon nitride film, following field oxide growth.
There appears to be a lack of consensus as to the factors that contribute to the pitting phenomenon. For example, although the pits are observed regardless of the process used to remove the silicon nitride layer, either by wet etch or by dry etch, some researchers have correlated the extent of pitting of the polysilicon layer to the soak time in the wet etchant.
Because the processes which remove the polysilicon may tend to transfer the pits into the underlying substrate, those concerned with the developments of integrated circuit technology have sought methods for reducing the size and number of pits.